1. Field of the Invention
The present invention is situated in the field of measuring technology (metrology) and concerns a method and a device for calibrating a measuring system that is applicable for determining spatial position and orientation of objects and that comprises a measuring device with a laser tracker and an opto-electronic sensor, a system computer, and an auxiliary measuring tool.
2. Description of Related Art
The term laser tracker or tracker in the present context is a device comprising means for generating an optical measuring beam, advantageously a laser beam, an optical system for aligning the measuring beam to a target point (e.g. cube edge prism) reflecting the measuring beam back to the measuring device in parallel, means for analysing the reflected measuring beam for determining the absolute or relative path length of the beam, and means for detecting the absolute or relative direction of the measuring beam. From the recorded data regarding beam path length and beam direction between laser tracker and reflector (target point), spatial co-ordinates of the reflector relative to the laser tracker can be calculated. The optical system of the tracker advantageously is equipped such that the measuring beam is able to automatically track a moving target point. Laser trackers of the Leica Geosystems company are available on the market. Other devices such as, for example, motorised theodolites comprising distance measurement means, also fall under the above definition.
The term opto-electronic sensor in the present context is a device equipped for creating an electronically evaluatable, two-dimensional image of a spatial arrangement of light spots. The opto-electronic sensor comprises a two-dimensional, light-sensitive array and a combination of lenses with an optical axis. It is, for example, a CCD or CID-camera or it is based on a CMOS-array. For evaluating the two-dimensional image, means are provided for identifying the imaged light spots, for determining the centers of gravity of the imaged light spots and for determining the image co-ordinates of these centers of gravity. From these coordinates, spatial angles between the optical axis of the sensor and the direction from the sensor to the light spots can be calculated.
The laser tracker and the opto-electronic sensor in the measuring device of the measuring system presented here are installed one on top of the other such that their positions relative to one another are fixed. For example, the laser tracker and sensor are rotatable together around an essentially vertical axis, and the sensor can be pivoted upwards and downwards independent of the laser tracker.
Measuring devices with a laser tracker and an opto-electronic sensor in accordance with the above definitions and with a system computer for carrying out the mentioned calculations and their utilization for determining spatial position and orientation of objects carrying light spots and reflectors belong to the state of the art. Measuring devices of this kind are available on the market (e.g., theodolite type T3000V/D of the Leica company). When using a measuring device of the named type for determining position and orientation of an object, at least three light points to be detected by the opto-electronic sensor and at least one reflector reflecting the measuring beam of the laser tracker are arranged on the object in known positions relative to the object. The light spots to be registered by the opto-electronic sensor may be active light sources (e.g. light-emitting diodes) or reflectors to be illuminated, wherein the light spots are equipped or arranged such that they can be identified in an unequivocal manner.
In many applications not the object, the position and orientation of which is sought, is measured itself but an auxiliary measuring tool, which belongs to the measuring system and which, for the measurement, is brought into a position relative to the target object, which position is mechanically defined or is determined during the measurement. From the measured position and orientation of the auxiliary measuring tool the sought position and, if so required, orientation of the target object can be calculated. Auxiliary measuring tools are, for example, so-called touch tools that are positioned on a target object with their contact point in contact with the target object. Light spots and reflector of the touch tool have exactly known positions relative to the contact point. Touch tools of the type are available on the market (e.g., Optrek 3-D Co-ordinate Measuring Stylus of Northern Digital Corp. Canada). However, the auxiliary measuring tool may also be a hand-held scanner for contact-free surface measurements. The scanner is equipped for distance measurement with the aid of a measuring beam and it comprises light spots and reflectors in known positions relative to direction and position of the measuring beam. A scanner of this kind is described, for example, is discussed in the publication EP-0553266 (Schulz).
It is obvious that, in a measuring system as described above, the laser tracker and the reflector (or the reflectors) of the auxiliary measuring tool, on the one hand, and the opto-electronic sensor and the light spots of the auxiliary measuring tool, on the other hand, represent separate measuring systems being coupled with one another through the relative arrangement of laser tracker and opto-electronic sensor in the measuring device and through the relative arrangement of light spots and reflector or reflectors on the auxiliary measuring tool. For correlating the two measuring systems, calibration is necessary. Calibration concerns the laser tracker and the opto-electronic sensor of the measuring device and the auxiliary measuring tool.